1. Technical Field
The present invention relates to a pushing block for a continuously variable transmission (CVT) belt and to a manufacturing method for the same, and more particularly, the present invention relates to a technique which can increase toughness in addition to hardness and can improve durability.
2. Related Art
A CVT belt is formed by continuously connecting a plurality of pushing blocks by a steel belt so as to form a circular shape, and as the pushing block, there has been provided a structure in which a hyper-eutectoid steel such as a material corresponding to SKS95 or the like is spheroidized and annealed and is thereafter formed, quenched, and tempered. At the time of quenching, a treatment is performed maintaining a temperature equal to or greater than an austenitizing temperature (about 750xc2x0 C.) for fifteen minutes or more, and in an actual operation, a maintaining time for thirty minutes to sixty minutes is typical.
A material structure of the pushing block manufactured in the manner described above demonstrates a martensitic structure having a non-dissolved carbide having an area percentage of about 0.4%.
However, since a heat treatment of the conventional CVT pushing block is performed mainly for the purpose of increasing hardness in order to ensure wear resistance, impact strength is low while hardness is about HRC60, and impact value (a Charpy U notch) is only about 20 J/cm2.
However, since the pushing block of the CVT belt requires toughness in addition to hardness due to being subjected to impacts, it is a problem to improve the impact strength of the pushing block for ensuring the durability at a time of making the CVT compact and applying the CVT to a high torque engine.
Accordingly, an object of the present invention is to provide a pushing block for a CVT belt which is provided with a sufficient toughness in addition to hardness and is superior in wear resistance and durability, and to provide a manufacturing method for the same.
The pushing block for the CVT belt (hereinafter referred to as a xe2x80x9cpushing blockxe2x80x9d) in accordance with the present invention is characterized by having a hyper-eutectoid structure overall and exhibiting a structure in which spheroidal cementite having an area percentage of 0.4% or more are dispersed in a matrix of a martensite.
In accordance with the pushing block having the structure described above, since the matrix is the martensite, a high surface hardness is provided, and since carbon is fixed to the spheroidal cementite and a concentration of the carbon in the matrix is low, toughness is improved. In this case, the effect improving toughness can be obtained when the area percentage of the spheroidal cementite in the structure is equal to or greater than 0.4%. Accordingly, in the pushing block in accordance with the present invention, it is possible to improve the durability in addition to the wear resistance. It should be noted that the area percentage of the spheroidal cementite in the structure is preferably equal to or greater than 1%, so that the impact strength is stable.
Furthermore, in accordance with the present invention, there is provided a manufacturing method for a pushing block comprising the steps of: performing a spheroidizing annealing to a hyper-eutectoid steel; and thereafter performing a quenching and a tempering, wherein the time for maintaining the pushing block at a temperature equal to or greater than an austenizing temperature in a heating operation for the quenching is set in a range of 5 to 40 minutes.
In accordance with the manufacturing method described above, it is possible to obtain a structure in which a spheroidal cementite is dispersed in a matrix of a ferrite due to the spheroidizing annealing. Carbon in the spheroidal cementite is dissolved in the austenite by heating and maintaining it at a high temperature. The manufacturing method in accordance with the present invention is characterized in that the time for maintaining the pushing block at the temperature equal to or greater than the austenizing temperature in the heating operation for the quenching is in a range of 5 to 40 minutes. In the case in which the time for maintaining the pushing block at a temperature equal to or greater than the austenizing temperature is less than 5 minutes, only a small amount of carbon is dissolved in the austenite, and the martensite is insufficiently generated in the quenching, therefore, it is impossible to obtain a necessary surface hardness. In contrast, in the case in which the time for maintaining the pushing block at the temperature equal to or greater than the austenizing temperature is more than 40 minutes, it is impossible to obtain an effect for improving toughness. It should be noted that the time for maintaining the pushing block at the temperature equal to or greater than the austenizing temperature is preferably not more than 15 minutes since the impact strength is stable.
In general, a normalized structure in a hyper-eutectoid steel consists of a mixed structure of a pearlite and a cementite. In such a structure, an austenitic transformation is quickly promoted, and most of the contained carbon is dissolved in the austenite. In contrast, in the present invention, since the structure consists of a ferrite and a spheroidal cementite, the austenitic transformation causes a diffusion of the carbon contained in the cementite into the austenite, and the diffusion is not quickly promoted, and it is believed that the spheroidal cementite remains in the austenite. However, the explanations above are theoretical, and therefore the present invention is not limited thereby.